The treatment of wells into oil and gas bearing formations to increase their productivity has long been known in the art. One of the most common methods of increasing productivity is to subject an oil or gas producing formation to a liquid, gas, or foam under a very high pressure. This liquid, gas or foam (known generally as a "fracturing fluid") causes ruptures or fractures in the oil or gas producing formation. A proppant material, such as sand, finely divided gravel, glass beads or the like, is introduced into these fractures, to keep them open. The fracture provides easier access to oil and gas bearing portions of the formation, thereby increasing the productivity of the well.
One of the most successful fracturing techniques has been that known as "foam fracturing". This process is described in U.S. Pat. Nos. 3,937,283 (Blauer and Durborow), and in 3,980,136 (Plummer and Johnson). Briefly, the foam fracturing process involves generating a foam of a desired "Mitchell quality", as described in the Blauer et al patent mentioned above, and pumping this foam down a well to be treated. The composition of the foam is such that the Mitchell foam quality at the bottom of the well will be in a range from about 0.53 to 0.99. Various gases and liquids may be used to create the foam, but the usual foams in the art are made from nitrogen and water, in the presence of a suitable foaming agent, such as for example, sodium dodecyl sulphate. The temperature and pressure at which the foam is pumped into the well is such that it will cause a fracture of the oil or gas bearing formations in the well.
Fracturing caused by the foam fracturing process is very effective, and has many advantages. For example, foam fracturing can be carried out with relatively low powered pumping equipment. Additionally, the foam comes out of the well easily when the pressure is removed from the well head, because the foam expands considerably when pressure is released. This means that the well can be "cleaned up" and returned to production promptly.
Despite its many advantages, there are also disadvantages to the present foam fracturing technique. It is known that the fractures formed by this technique are long and thin. Additionally, the foam cannot support and carry high amounts of proppant, and practical proppant values are of the order of two pounds of sand (when sand is used as a proppant) per gallon of foam of the desired Mitchell quality. Because of the long, thin nature of the fractures, and the relatively low amounts of proppant, fractures formed by a foam fracturing process may not stay open. Additionally, the long, thin fractures are not efficient for delivery of oil or gas from the fractured production zone.
Accordingly, it would be desirable to develop a process which had the advantages of foam fracturing, such as easy clean out and relatively low pumping power requirements, but which provided a fracture which was thicker and filled with more proppant than can be delivered conveniently by the foam fracturing method.